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OPA695: Output abnormal when temperature increases

Jiandong Liu
Intellectual 1801 points
Part Number: OPA695
Other Parts Discussed in Thread: OPA690

Hi expert,

My customer use OPA695 and find some of them output abnormal when temperature increases. Please see schematic and waveforms in attached pdf.

OPA695测试.pdf

Regards and thanks.

Jiandong

  • 0
    Guru 47485 points

    I am surprised it works at all, you are trying to make a current source with a current feedback part. The total feedback Transimpedance (that sets stability)  includes the transconductance of the FET, that may well vary over temperature - the pin equivalent OPA690 is a drop in alternate to try, not as as fast, but pretty good on slew rate. 

  • 0 in reply to Michael Steffes
    TI__Intellectual 1801 points

    Hi Michael,

    Thanks for your reply.

    1. Could you explain why current feedback part can't make a current source?

    2. As you mentioned, feedback transimpedance varies with temperature causes unstable. Do you think if can make OPA695 stable by increasing the feedback resistors? 

    Regards and thanks.

    Jiandong

  • +1 in reply to Jiandong Liu
    Guru 47485 points

    Well if you force it, a CFA can probably do this - but to buy phase margin over the FET transconductance range and load (the FET Zo and load form a thevinin equivalent source Z to the feedback network) you would want to raise the feedback R until you are stable over temp - which might just bandlimit you back to where a part like the OPA690 is just as adequate. yes, make R4 about 750ohm and see if your situation improves.